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It is widely believed that a good principal is the key to a successful school. No Child Left Behind encouraged the replacement of the principal in persistently low-performing schools, and the Obama administration has made this a requirement for schools undergoing federally funded turnarounds. Foundations have invested millions over the past decade in New Leaders for New Schools, an organization that recruits nontraditional principal candidates and prepares them for the challenges of school leadership. And the recently launched George W. Bush Institute is making the principalship a focus of its activities. Yet until very recently there was little rigorous research demonstrating the importance of principal quality for student outcomes, much less the specific practices that cause some principals to be more successful than others. As is often the case in education policy discussions, we have relied on anecdotes instead. This study provides new evidence on the importance of school leadership by estimating individual principals’ contributions to growth in student achievement. Our approach is quite similar to studies that measure teachers’ “value added” to student achievement, except that the calculation is applied to the entire school. Specifically, we measure how average gains in achievement, adjusted for individual student and school characteristics, differ across principals—both in different schools and in the same school at different points in time. From this, we are able to determine how much effectiveness varies from one principal to the next.